This invention relates to sulfonium compounds and, more particularly, to photosensitization of aromatic sulfonium compounds.
Sulfonium salts are known to absorb ultraviolet light only at wavelengths below about 300 nm, and their absorption spectra is essentially independent of the anion portion of the molecule. This limited spectral response is accordingly a serious inherent limitation with respect to their use as photoinitiators in photosensitive compositions. For example, in the field of graphic arts, such as in the preparation of printing plates, a photographic film negative is disposed between the lamp and the photosensitive plate. This film negative, however, prevents the transmission of appreciable light of wavelengths below about 340 nm. Furthermore, when photosensitive compositions are used as photocurable coatings, light of wavelengths below about 300 nm. decreases in intensity as it passes through the coating, due to absorption of the light by the monomeric compounds, binders, etc., present in the coating.
Although there are dozens of known classes of dyes and other materials which can be used to broaden the spectral response of photosensitive compositions, it has not been possible to predict which materials would be useful in sensitizing aromatic sulfonium salts. In fact, it has been found that only select materials are useful in sensitizing aromatic sulfonium salts. For example, a well known and widely used class of energy transfer sensitizers, known as triplet sensitizers, e.g., benzophenone, acetophenone, etc., are not significantly effective for sensitizing aromatic sulfonium salts.